Experiments are designed to provide fundamental insight into the metabolic processes whereby saturated and unsaturated fatty acids may affect living cells. We will use fatty acid auxotroph mutants of E. coli and S. cerevisiae as model systems for prokaryotic and eukaryotic cells. Quantitative analyses of cellular membrane lipids allow assignment of the degree to which nutrient unsaturated acids contribute to the physical properties of the membrane in an additive, non-selective manner as often ascribed to fluidity. Futher quantitative measurement of cell growth in response to limiting amounts of nutrient allow assignment of the degree to which selective, non-additive effects of the specific fatty acid influence cellular function. These latter effects represent important new ways in which fatty acids can affect cell biochemistry and have been seen in examples of induction of new enzymes, initiation of cell division and mitochondrial replication. The selective regulatory action of certain unsaturated fatty acids will be examined in terms of its effect on specific enzyme activities and how they are affected by metabolite regulation and cellular cAMP levels. The results will be applied to more complex mammalian cell systems to help interpret the possible impact of various dietary lipids upon human physiology.